Successful treatment of hypertension is important for many reasons. For example, successful treatment of hypertension has significant clinical benefits in preventing or limiting conditions caused by or exacerbated by hypertension, such as renal disease, arrhythmias, and congestive heart failure, to name a few. While drug therapy can be used to treat hypertension, it is not always successful. Some people are resistant to drug therapy treatment or experience significant side effects from drug therapy treatment.
Hypertension can be treated by inactivating conduction of the renal nerves surrounding the renal artery. Sympathetic renal nerve activity plays a significant role in the initiation and maintenance of hypertension. When the brain perceives increased renal nerve activity, signaling low blood volume or a drop in blood pressure, it compensates by increasing sympathetic nerve activity to the heart, the liver, and the kidneys, which results in increased cardiac output; insulin resistance; and most importantly, increased renin production by the kidneys. Renin stimulates the production of angiotension, which causes blood vessels to constrict, resulting in increased blood pressure and stimulates the secretion of aldosterone. Aldosterone causes the kidneys to increase the reabsorption of sodium and water into the blood, increasing blood volume thereby further increasing blood pressure.
It has been established for years that surgically cutting renal nerves results in a decrease in blood pressure and water retention to normal levels; thereby allowing the patients' heart, liver, and kidneys to also return to healthier functioning. It has also been shown a disruption of the renal nerves has no serious ill effects. However, surgically cutting the renal nerves requires a major surgical procedure with risks of undesirable side effects. It would be desirable to produce the same result without major surgery.
In order to explain the difficulties associated with accomplishing this task without causing other damage, the anatomy of the renal arteries and nerves will be described now. Shown in FIG. 1 is an illustration of the renal nerves 8 that surround the renal artery 10, which is connected to the kidney 6. The sympathetic renal nerves 8 include both the afferent sensory renal nerves from the kidney 6 to the brain and the efferent sympathetic renal nerves from the brain to the kidney 6. In addition, FIG. 2 shows a cross-section of a renal artery 10. The renal artery wall includes layers: the intima 3, which includes an inner single layer of endothelial cells; the media 5, which is in the center of the artery wall; and the adventitia 4, which is the outside layer. Also shown are the renal nerves 8 that lie within the adventitia 4, on the surface of the renal artery 10, and adjacent to the renal artery 10. As can be seen from these two figures, the renal nerves 8 surround the renal artery 10. Different individuals have the renal nerves 8 in different locations around the renal artery. Thus, the renal nerves may be at different radial distances R from the central axis A of the renal artery, and also may be at different locations around the circumference C of the renal artery. It is not practical to locate the renal nerves by referring to anatomical landmarks. Moreover, it is difficult or impossible to locate individual renal nerves using common in vivo imaging technology.
The inability to locate and target the renal nerves 8 makes it difficult to disconnect the sympathetic renal activity using non-surgical techniques without causing damage to the renal artery 10 or causing other side effects. For example, attempts to apply energy to the renal nerves can cause effects such as stenosis, intimal hyperplasia, and necrosis. Other side effects can include thrombosis, platelet aggregation, fibrin clots and vasoconstriction. In addition, the inability to target and locate the renal nerves 8 makes it difficult to ensure that sympathetic renal nerve activity has been discontinued enough to achieve an acceptable therapeutic treatment.
U.S. Pat. No. 7,617,005 suggests the use of a radio frequency (“RF”) emitter connected to a catheter, which is inserted in the renal artery. The RF emitter is placed against the intima and the RF energy is emitted to heat the renal nerves to a temperature that reduces the activity of renal nerves which happen to lie in the immediate vicinity of the emitter. In order to treat all the renal nerves surrounding the renal arteries, the RF emitter source must be repositioned around the inside of each renal artery multiple times. The emitter may miss some of the renal nerves, leading to an incomplete treatment. Moreover, the RF energy source must contact the intima to be able to heat the renal nerves, which may cause damage or necrosis to the single layer endothelium and the intima, potentially causing intimal hyperplasia, renal artery stenosis, and renal artery dissection.
The '005 patent also suggests the use of high-intensity focused ultrasound to deactivate the renal nerves. The described high-intensity focused ultrasound energy source assertedly emits ultrasound energy in a 360° pattern around the axis of the renal artery, and does not need to contact the intima 3. However, the high-intensity focused ultrasound source applies concentrated energy in a thin focal ring surrounding the artery. It is difficult or impossible to align this thin ring with the renal nerves because it is difficult or impossible to visualize and target the renal nerves with current technology, and because the renal nerves may lie at different radial distances from the central axis of the renal artery. The latter problem is aggravated in patients who have renal arteries with large variations in shape or thickness. Moreover, the thin focal ring can encompass only a small segment of each renal nerve along the lengthwise direction of the nerves and artery. Since nerves tend to re-grow, a small treatment zone allows the nerves to reconnect in a shorter period of time.
For many years ultrasound has been used to enhance cell repair, stimulate the growth of bone cells, enhance delivery of drugs to specific tissues, and to image tissue within the body. In addition, high-intensity focused ultrasound has been used to heat and ablate tumors and tissue within the body. Ablation of tissue has been performed nearly exclusively by high-intensity focused ultrasound because the emitted ultrasound energy is focused on a specific location to allow precise in-depth tissue necrosis without affecting surrounding tissue and intervening structures that the ultrasound energy must pass through.
U.S. Pat. No. 6,117,101, to Diederich, discusses use of highly collimated ultrasound energy rather than high intensity focused ultrasound for ablating tissue to create a scar ring within the pulmonary vein for blocking the conduction of electrical signals to the heart.
US Patent Publication No. 20100179424 (application Ser. No. 12/684,067), the disclosure of which is incorporated by reference herein, uses unfocused ultrasound for the treatment of mitral valve regurgitation. In the '474 Publication, unfocused ultrasound energy is used to heat and shrink the collagen associated with the mitral annulus. This apparatus uses an inflatable balloon in order to place the ultrasound transducer into the correct location, thereby targeting the mitral annulus. In this apparatus, a part of the balloon contacts the tissue to be heated.